polylineplot with relaxed y-clamping

[saintmatthieu]

Needed for audacity/audacity#10223

This effect seems to be the first use-case where points have to be rendered offscreen - observe in the screenshot how the line at x=0 is below the bottom edge.

FWIW, I manually tested after my changes the two other places where these are used: Audacity's and MSS's volume automations. I couldn't find a regression there.

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• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

Build configuration

audacity: saintmatthieu/audacity/10223-port-filter-curve-eq
audacity platforms: linux_x64
musescore: saintmatthieu/MuseScore/polylineplot-with-relaxed-y-clamping
musescore platforms: linux_x64

[saintmatthieu]

/build

[saintmatthieu]

@mathesoncalum and @kryksyh as reviewers please

[kryksyh]

/build

[github-actions]

Build dispatched: https://github.com/musescore/muse_framework/actions/runs/25788578143